In phonetics, a stop, also known as a plosive, is an oral occlusive, a consonant in which the vocal tract is blocked so that all airflow ceases. The occlusion may be made with the tongue blade ([t], [d]) or body ([k], [ɡ]), lips ([p], [b]), or glottis ([ʔ]). Stops contrast with nasals, where the vocal tract is blocked but airflow continues through the nose, as in /m/ and /n/, and with fricatives, where partial occlusion impedes but does not block airflow in the vocal tract.

The terms stop, occlusive, and plosive are often used interchangeably. Linguists who distinguish them may not agree on the distinction being made. The terms refer to different features of the consonant. "Stop" refers to the airflow that is stopped. "Occlusive" refers to the articulation, which occludes (blocks) the vocal tract. "Plosive" refers to the release burst (plosion) of the consonant.

Either "occlusive" or "stop" may be used as a general term covering the other together with nasals. That is, 'occlusive' may be defined as oral occlusives (stops/plosives) plus nasal occlusives (nasals such as [m], [n]), or 'stop' may be defined as oral stops (plosives) plus nasal stops (nasals). Ladefoged and Maddieson (1996) prefer to restrict 'stop' to oral occlusives. They say,

what we call simply nasals are called nasal stops by some linguists. We avoid this phrase, preferring to reserve the term 'stop' for sounds in which there is a complete interruption of airflow.[1]

If a term such as 'plosive' is used for oral obstruents, and nasals are not called nasal stops, then a stop may mean the glottal stop; 'plosive' may even mean non-glottal stop. In other cases, however, it may be the word 'plosive' that is restricted to the glottal stop. Note that, generally speaking, stops do not have plosion (a release burst). In English, for example, there are stops with no audible release, such as the /p/ in apt. However, pulmonic stops do have plosion in other environments.

In Ancient Greek, the term for stop was ἄφωνον (áphōnon),[3] which means "unpronounceable", "voiceless", or "silent", because stops could not be pronounced without a vowel. This term was calqued into Latin as mūta, and from there borrowed into English as mute.[4]Mute was sometimes used instead for voiceless consonants, whether stops or fricatives, a usage that was later replaced with surd, from Latin surdus "deaf" or "silent",[5] a term still occasionally seen in the literature.[6] For more information on the Ancient Greek terms, see Ancient Greek phonology § Terms for consonant letters.

All languages in the world have stops,[7] and most have at least the voiceless stops [p], [t], and [k]. However, there are exceptions: Colloquial Samoan lacks the coronal[t], and several North American languages, such as the northern Iroquoian and southern Iroquoian languages (i.e., Cherokee), lack the labial[p]. In fact, the labial is the least stable of the voiceless stops in the languages of the world, as the unconditioned sound change [p] → [f] (→ [h] → Ø) is quite common in unrelated languages, having occurred in the history of Classical Japanese, Classical Arabic, and Proto-Celtic, for instance. Formal Samoan has only one word with velar[k]; colloquial Samoan conflates /t/ and /k/ to /k/. Ni‘ihauHawaiian has [t] for /k/ to a greater extent than Standard Hawaiian, but neither distinguish a /k/ from a /t/. It may be more accurate to say that Hawaiian and colloquial Samoan do not distinguish velar and coronal stops than to say they lack one or the other.

In aspirated stops, the vocal cords (vocal folds) are abducted at the time of release. In a prevocalic aspirated stop (a stop followed by a vowel or sonorant), the time when the vocal cords begin to vibrate will be delayed until the vocal folds come together enough for voicing to begin, and will usually start with breathy voicing. The duration between the release of the stop and the voice onset is called the voice onset time (VOT) or the aspiration interval. Highly aspirated stops have a long period of aspiration, so that there is a long period of voiceless airflow (a phonetic [h]) before the onset of the vowel. In tenuis stops, the vocal cords come together for voicing immediately following the release, and there is little or no aspiration (a voice onset time close to zero). In English, there may be a brief segment of breathy voice that identifies the stop as voiceless and not voiced. In voiced stops, the vocal folds are set for voice before the release, and often vibrate during the entire hold, and in English, the voicing after release is not breathy. A stop is called "fully voiced" if it is voiced during the entire occlusion. In English, however, initial voiced stops like /#b/ or /#d/ may have no voicing during the period of occlusion, or the voicing may start shortly before the release and continue after release, though word-final stops tend to be fully voiced: In most dialects of English, the final g in the bag is likely to be fully voiced, whereas the initial b will only be voiced during part of its occlusion. Initial voiceless stops, like the p in pie, are aspirated, with a palpable puff of air upon release, whereas a stop after an s, as in spy, is tenuis (unaspirated). When spoken near a candle flame, the flame will flicker more after the words par, tar, and car are articulated, compared with spar, star, and scar. In the common pronunciation of papa, the initial p is aspirated whereas the medial p is not.

In a geminate or long consonant, the occlusion lasts longer than in simple consonants. In languages where stops are only distinguished by length (e.g., Arabic, Ilwana, Icelandic), the long stops may be held up to three times as long as the short stops. Italian is well known for its geminate stops, as the double t in the name Vittoria takes just as long to say as the ct does in English Victoria. Japanese also prominently features geminate consonants, such as in the minimal pair 来た kita 'came' and 切った kitta 'cut'.

Note that there are many languages where the features voice, aspiration, and length reinforce each other, and in such cases it may be hard to determine which of these features predominates. In such cases, the terms fortis is sometimes used for aspiration or gemination, whereas lenis is used for single, tenuous, or voiced stops. Be aware, however, that the terms fortis and lenis are poorly defined, and their meanings vary from source to source.

Simple nasals are differentiated from stops only by a lowered velum that allows the air to escape through the nose during the occlusion. Nasals are acoustically sonorants, as they have a non-turbulent airflow and are nearly always voiced, but they are articulatorily obstruents, as there is complete blockage of the oral cavity. The term occlusive may be used as a cover term for both nasals and stops.

A prenasalized stop starts out with a lowered velum that raises during the occlusion. The closest examples in English are consonant clusters such as the [nd] in candy, but many languages have prenasalized stops that function phonologically as single consonants. Swahili is well known for having words beginning with prenasalized stops, as in ndege 'bird', and in many languages of the South Pacific, such as Fijian, these are even spelled with single letters: b [mb], d [nd].

A postnasalized stop begins with a raised velum that lowers during the occlusion. This causes an audible nasal release, as in English sudden. Russian and other Slavic languages have words that begin with [dn], which can be seen in the name of the Dnieper River.

Note that the terms prenasalization and postnasalization are normally used only in languages where these sounds are phonemic: that is, not analyzed into sequences of stop plus nasal.

A fortis stop (in the narrow sense) is produced with more muscular tension than a lenis stop (in the narrow sense). However, this is difficult to measure, and there is usually debate over the actual mechanism of alleged fortis or lenis consonants.

There are a series of stops in the Korean language, sometimes written with the IPA symbol for ejectives, which are produced using "stiff voice", meaning there is increased contraction of the glottis than for normal production of voiceless stops. The indirect evidence for stiff voice is in the following vowels, which have a higher fundamental frequency than those following other stops. The higher frequency is explained as a result of the glottis being tense. Other such phonation types include breathy voice, or murmur; slack voice; and creaky voice.